It would be useful if you could specify the exact time of a channel update - eg capture share prices after markets have closed.

This was on the to-do list for a long time but I never got around to it since there didn't seem to be much demand.

In this case, is it really necessary? For example, if the market closes at 5pm and your channel is set to update every 5 minutes, in the worst case you'll get the closing prices at 5:05pm. They won't change after 5pm so the only issue is a slight delay.
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If you really want to control update times, you can do it with channel blackouts (General tab in the Program Options dialog), although this will affect all channels.